News

Why spite?

Spite, in the words of Merriam and Webster, is “the desire to hurt, annoy, or offend someone.” But I like this definition better: “Spite [is] the shady relative of altruism.” If altruism is costly behavior that helps another, spiteful characters pay to harm another.

This is how Northeastern assistant professor of philosophy Rory Smeaddescribes the vengeful behavior in a paper released earlier this year in the journal Evolution.

Turns out big sisters and Draco Malfoy aren’t the only creatures that participate in the activity. Birds do it. Bacteria do it. Even adolescent yellow-rumped caciques do it (young males of the species harass the females and fledglings for no apparent reason, costing themselves time and energy and sometimes getting themselves hurt when the girls fight back).

But it’s curious that we observe this behavior in nature: it’s expensive (remember the time Malfoy got turned into a ferret for being a jerk?) and it doesn’t seem to have much benefit to the perpetrator. ”Spite hurts both the actor and the recipient, so it would generally be better not to be spiteful,” said Smead. So what do behaviors that cost resources and are apparently useless still doing in ecology after billions of years of selection? Shouldn’t this kind of thing have run its course by now?

“The costs here, in evolutionary biology, are understood in terms of fitness, or reproductive potential,” explained Smead. “Having offspring is important for evolution, but so is having more offspring than others. The key for the evolution of spite is that an organism might be able to sacrifice some of its own fitness in order to gain a relative advantage over others. In other words, spite involves a trade-off: lower your own fitness, but lower the fitness of everyone else more.”

Back in the 1960s a guy named Bill Hamilton figured out that this mechanism would allow spite to evolve in small populations, where a little bit of spite can go a long way. The idea was dropped because everyone assumed that these small populations were on their way to extinction anyway, so it didn’t matter much. Those observations of spite found in nature were chalked up to misidentified cases of selfishness–that’s when harming another individual costs nothing or even benefits the actor.

But in their new research, Smead and his collaborator Patrick Forber of Tufts University, revisited Hamilton’s old models and have come to some new conclusions. Instead of looking at static populations like Hamilton did, the duo looked at dynamic ones, where population size and growth rate were constantly changing, which is much closer to the real world scenario anyway. Now they found that spite could evolve and persist even in larger populations.

“In the models we’re looking at all that you need is the right setting (small enough population combined with a low cost-to-harm ratio for spiteful behavior) and a single mutation that introduces spite will start spreading in the population.”

At a certain point it even starts to be good to be bad, because the spite ends up acting to regulate population size, something that can be very important when resources are limited. So, in fact, spite can actually create the very circumstances necessary for it to persist, write the authors.